Calibrating probes are very easy, Just make sure you have new calibration fluid which most places who sell ghl stuff would be able to get for you. Just go to the menu and click on calibration and follow steps on profilux screen. Just make sure you don't mix salt water or any water with calibration fluid. also make sure fluid is same temp as tank water. Just take the bottles and leave in a plastic bag in your sump for 30 mins to match same temp and you are good to go.